Hello, my first bird had a poo hanging from her a while back. I did take it off for her but I think they can do it themselves, I just didn’t want her to accidentally swallow any bacteria. One poo is usually nothing to worry about as long as they’re otherwise acting normal. Definitely keep an eye on them for a while though!
If more accumulates, if you see partially digest seed in it or if they have diarrhea definitely bring them to an avian vet. This happened to my second parakeet and it turns out she has Avian Gastro Yeast, (easily treatable if caught early.)
Your bird could also just need a little more water. Since budgies drink such a little amount, my vet suggested rinsing veggies or fruits they like with water before giving it to them!